Fuel cell technology is a relatively recent development in the automotive industry. It has been found that fuel cell power plants are capable of achieving efficiencies as high as 55%. Furthermore, fuel cell power plants emit no harmful by-products which would otherwise contribute to atmospheric pollution.
In order to prevent leakage of the hydrogen fuel gas and oxygen gas supplied to electrodes and prevent mixing of the gases in a fuel cell, a gas-sealing material and gaskets are arranged on the periphery of the electrodes, with the polymer electrolyte membrane sandwiched there between. The sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane to form a membrane and electrode assembly (MEA). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and electrically connecting adjacent MEAs in series. A portion of the separator plate, which is disposed in contact with the MEA, is provided with a gas passage for supplying hydrogen fuel gas to the electrode surface and removing generated water.
During fabrication of a fuel cell, the polymer electrolyte membrane of each MEA is produced in roll form under tension. The polymer electrolyte membrane has a high water uptake capability. Therefore, when wet, the membrane will expand in all three directions, although not proportionally. The membrane will shrink in all three dimensions upon subsequent drying.
As shown in FIG. 1 for example, the dimensional change of the membrane is irreversible after the first wet/dry cycle, since after drying, the membrane shrinks to a size which is smaller than the size of the membrane before the membrane was wetted for the first time. After two wet/dry cycles, however, the dimensional changes in the subsequent cycles are shown to be reversible since the membrane shrinks to the same size after completion of each wet/dry cycle. The irreversible component of the dimensional change is caused by the residue stress. The shrink rate in the X-Y dimensions is about 1-15%, as shown in FIG. 1 for example.
In a conventional MEA, the polymer electrolyte membrane is assembled into the MEA. In a fuel cell, multiple MEAs are assembled in fuel cell stacks, with one MEA per cell. One of the major failure modes for most fuel cell stacks are due to H2 crossover to the cathode side across the polymer electrolyte membrane, due to the presence of pinholes in the membrane. SEM analysis on the crossover locations has revealed that the membrane pinholes are caused by mechanical stresses such as tensile stress applied to the membrane. These stresses are related to the fuel cell operating conditions, especially to the relative humidity (RH) swing and cycling rates. Each MEA, having never been subjected to a wet/dry cycle, is assembled into a fuel cell under a relative humidity (RH) of typically about 20-60%. During fuel cell operation, each MEA will undergo wet and dry cycles in which the RH can be as high as 150%.
After the first wet/dry cycle of fuel cell operation, the MEA shrinks in size. However, the shrinking movement of the membrane is resisted by stack seals which clamp the MEA edges to a fixed dimension. This exerts stress on the polymer electrolyte membrane, causing the formation and propagation of pinholes in the membrane. Therefore, a method is needed to pre-treat an MEA prior to assembly of the MEA into a fuel cell stack in order to eliminate or reduce the irreversible dimensional changes of a polymer electrolyte membrane throughout wet/dry cycles during operation of the fuel cell. By eliminating or reducing the irreversibility of the dimensional changes of the polymer electrolyte membrane minimizes tensile stress and reduces the formation and propagation of pinholes in the membrane, leading to increased MEA lifetime.